Corrosion resistant coating



Patented Apr. 18, 1944 Joseph B.

Ohio

Brennan and Leona Marsh, said Marsh anignor to said Brennan Euclid,

No Drawing. Application December 2, 19, Serial No. 243,542

8 Claims.

Our invention relates to the forming of insulating and corrosionresistant oxide coatings of film forming metals such as aluminum,magnesium, tantalum, zinc, etc., and their alloys.

We have for an especial object of our invention the formation of a noveltype of such film or coating which evinces great capillarity in that itis highly absorptive, flexibility in that it can be distorted withoutcracking excessively, stability in that it resists chemical andelectrochem ical disintegration, uniformity in that it is of equalthickness even on irregular shaped bodies.

Another object of this invention is the formation of a smooth althoughsomewhat porous coat ing which is electrically burned onto the aluminum.Another object is the production of a tenacious corrosion resistantcoating at higher voltages than have been used in past practice.

A further object is the formation of a smooth. microscopically porous,adherent coating which is relatively and permanently, resistant tosevere atmospheric conditions.

We have for a further object'cf our invention the simplification ofmethods in producing such oxide coating by methods heretofore notdisclosed.

It is ordinary practice to secure contact with pieces to be anodized orelectrolytically coated by attachment on a so-called rack used to exertpositive pressure on each piece to be so anodized or coatedindividually. This ordinary practice requires the individual mounting ofeach piece on the so-called rack .prior to the introduction thereof intothe anodizing bath.

By our method such individual mounting is unnecessary since we obtainand retain piece to piece contact during fil formation withoutindividual mounting on a rack or otherwise. Furthermore even thoughpiece to piece contact is very light and even after the pieces arefilmed, we obtain conduction through their film to adjacent piecesbecause of our peculiar novel method of forming electrolytically. Thusanodic films can be formed by our method in a plating barrel havingelectrodes which will not disintegrate as of aluminum.

In forming this coating on metals such as aluminum we preferably usedirect current, however, our work has shown that considerablealternating current ripple does not affect the formation nor the qualityof the coating. The voltage is governed by the sparking voltage of theelectrolyte used. Many of the electrolytes which we found to produce thequality of corrosion resistant coating desired have fairly definitesparking voltages at a voltage higher than the usual electrolyte usedfor anodically forming a corrosion resistant coating on aluminum.

Apparently any metallic conductive material may be used as the cathode.We obtained good coatings using both aluminum anode and cathode, thealuminum cathode appearing bright as originally after a number of runs.We also found that copper and many other metals as the oathode could beused with an aluminum anode to produce excellent coatings.

Although the temperature does not afi'ect the quality of the corrosionresistant coating of the aluminum when varied from twenty-five to abouteightly degrees centigrade we prefer to use a moderately low temperatureto reduce the evaporation of the electrolyte and its tendency tosolidify. Temperature control may be afiected by liquid conduction coilsin the electrolyte thermostatically controlled.

In our electrolytes which produce satisfactory coatings at or above thesparking voltage of the respective electrolyte we include organic,inorganic, and mixed potential condensation products. We have foundorganic potential condensation products of the formaldehyde type to beparticularly useful for our purpose in so much that the coating can beformed with little regard to temperature, or purity of the chemicalsused. Since an urea-formaldehyde condensation product containing filmforming salts is completely miscible with water -or alcohol, we havefound this condensation product particularly adaptable for theproduction of our corrosion resistant coat- 8s.

Our investigation has shown that electrolytes of the artificial resin orcondensation product type containing oxygen hearing or film forming saltand/or acid as disclosed in our U. S. Patent No. 2,095,966 areparticularly useful for anodically forming corrosion resistant films onaluminum, the formation preferably being at or in excess of the sparkingvoltage.

The following formula is cited as an example of one of theurea-formaldehyde formulae used for anodically coating aluminum.

Parts Urea 200 Formaldehyde (40% solution by volume) Borlc acAmmonium'hydroxid e (spjgr. 0.9) 112 Phenol-formaldehyde condensationproducts containing boric acid and ammonium hydroxide, hydroxide, etc.also form boric acid and sodium by dipping or spraying or centrifugingor vacuum impregnating the article to be coated, form a coating afterburning in with an are light or simlar light minutely thin but affordinga tough, flexible continuous insoluble layer over the metal previouslyprotected by the anodic treatment. Dyes may be dissolved in thesensitized solution for coating the metal, water soluble dyes being usedin the albumen type of sensitized coatings. We have also applied thedyes directly to the anodized metal and obtained a uniform pleasingcolor. The dyes can be applied after light exposing the sensitizedcoating, however, we prefer to dissolve the dyes in the sensitizedsolution before sealing the oxidized metal surface with this solution.Dilute solutions of greasy inks have been applied to the light exposedsensitised coating, resuling in a coated metal which wears exceedinglywell under excessive corrosion conditions. A dilute solution of latexcontaining a small percentage of sensitizing compound as a chromateafter being light exposed results in a tenacious flexible film notattacked by many corrosive materials.

Various aluminum alloys have been anodically treated at or in excess ofthe sparking voltage simultaneously by our method. We have found thataluminum of greater than 99.9% purity as (28) and the aluminum alloys17S, Al 7S, and 248 were equally well coated in the same bath for thesame length of time. This simultaneously anodizing of different alloysin the same bath has not been successful heretofore to our knowledge.Another indication of stability, uniformity and continuity of our oxidefilm is characterized in that metal pieces so coated may be brought intophysical contact with one another without breakdown or sparking over orshorting when voltage is applied'thereto. If these oxide coatings areimpregnated as in a, vacuum with a dielectric such as a wax or varnishthen such oxide coatings so impregnated withstand many thousand' volts.The above indivates the complete conversion of surface metal to film oroxide form with no points of high conduction of the surface anddistinguishes the results of our Process.

Heating and or vacuum drying of our films prior to applyingseals isdesirable in that it extends corrosion resistance characteristics.Drying prior to centrifugal impregnation also adds life to articles socoated because of increased penetration. There are certain cases wherecentrifugal impregnation may be combined with dry ing in vacuo orotherwise to advantage.

- Furthermore formation in motion or with agitation of electrolyte or acombination thereof may in certain applications be patricularlyadvantageous.

Bakin in of impregnating compounds to complete resiniilcation as forinstance where a Glyptol cement or Bakelite cement is used as impregvnation media it may be advantageous to harden and set same byapplication of heat thereto.

Other aids to penetration of impregnating compounds as by rolling inmechanically are contemplated.

Tumbling of articles in groups during formation and impregnation is alsocontemplated.

A corrosion resistant coating may also be obtained by forming slightlyabove the breakdown voltage of the film in a solution of boric acid anda borate salt. I

Where formation electrolytes such as potential resins are used articlesmay be removed therefrom after formation and'heat and pressure appliedthereto thereafter resulting in that the potential resin clingingthereto is converted over by said healt and pressure to a stable resincoating and sea Where objects of irregular shape are to be cured afterhaving been removed from potential resin formation electrolyte thepressure ma be applied with heat in a sealed pressure chamber filledwith gas or other non-setting liquid under pressure. This system ofapplying heat and pressure to irregularly shaped resinous coatedarticles may be used to advantage extensively and is novel. The articleswhen removed from said pressure chamber are impregnated thoroughly andthe resin is set.

One outstanding advantage to sealing methods disclosed by us herein isthat we'seal with compounds which after sealing are rendered insolubleas for instance in water. This is not true of heretofore used chromateseals.

By our formation method whereby piece to piece conduction can be had dueto current leakage at high voltages continuous feeding and forgroundingit and in the other baths if desired similarly or if desired they ortheir containers may be insulated from ground.

As an explanation of the peculiar results secured by us in our formationwhereby we secure an extremely uniform coating both as to color andcorrosion resistant properties we believe that these results are due tothe fact that the electrolytes used by us are of extremely uniformresistance characteristics where they are in contact with the surfacesbeing anodized giving an extremely uniform field efi'ect adjacent thegas layers deposited on the surfaces.

The higher voltage used by us are probably of extreme importance becauseof the fact that since the field eflect gained therefrom in the vicinityof the formed gas layer is more intense it ottercomes the variablesexistent due to minute metallie impurities present in the surface of themetal being anodized resulting in that our film builds over these minuteareas of metallic impurit at least partially sealing these over with theinert oxidelayer. In any case by our method the film layer seems to beone of perfect continuity and of completely uniform whiteness and oflower dielectric leakage than ordinary films. ,Our film isalso moreabsorbtive than any others known and tested.

We prefer to form at high voltagessay from 250 to 500 volts or over as apeak for corrosion resistant coatings.-

It is this apparent ability of our system to cause the oxide layer toextend itself surfacewise on articles being anodized which furthermoreaccounts for. the fact vthat even .where articles are being anodizedunder our: system in contact with ortouching .one another there is nogap in theoxide layer at' suchpoints and no resultant contactscanisevident where the pieces being anodized wereftouching Anotherpossible explanation; may be that using higher voltage electrolytes thegrowth of the oxide layer .at contact points is not blocked because ofthe high field intensity at these points forcing the transformation ofthe exposed metals at these points.

UndoubtedLv the looseness of contact with the source of positive currentalso assists film formation at the contact points because it permits theelectrolyte to fill in or flow in between the articles being anodizedand adjacent articles in a minute layer and also between articles nearthe source of positive current and said source. Thus although wedescribe articles being anodized as in contact with a source of positivecurrent none the less it must be understood that a layer of electrolyteof minute thickness intervenes at said points of contact.

From the above explanation and reasoning we believe that we havedeveloped the first correct method of anodizing articles immersed in afilm forming electrolyte because according to ordinary practice wherebymechanical contact pressure is ordinarily exerted on articles being soanodized we have eliminated this false and unnecessary mechanicalpressure and by so doing have elimihated the scars or marks ordinarilyon articles so anodized from articles anodized by our. method.

We find that it is mdst advantageous to operate near or preferably at orabove the sparking voltage of the electrolytic system employed but donot wish to be limited thereby since a longer exposure to electrolysisunder the methods disclosed and with the electrolytes of the classspecified will in some cases give good results.

Our method of mass formation without a mechanical pressure electricalcontact to. articles being formed and our method of developing throwingpower in electrolytic oxidation to produce porous corrosionresistant-coatings is novel. Ordinary high voltage films on aluminum areshiny, hard and brittle and not highly absorptive or white as ours are.

We have also/found that electrolytes may be used comprising a ketonesuch as acetone and formaldehyde and an ionizing film forming salt suchas boric acid and ammonium hydroxide to give suitable results.

We claim:

l. The method oi. forming corrosion resistant coatings on aluminum whichincludes the steps of subjecting the aluminum to electrolysis as ananode in an aqueous solution of a potential condensation product of ureaand formaldehyde having boric acid and ammonium hydroxide added theretoand carrying out the electrolysis at a voltage such that sparking takesplace on the surface of the aluminum.

2. The method of forming corrosion resistant, coatings on aluminum whichincludes the steps of subjecting the aluminum to electrolysis as ananode in an aqueous solution of a potential I condensation product offormaldehyde and a material of the class consisting of urea and phenol,and a film forming electrolyte, carrying out the electrolysis at avoltage such that sparking takes place on the surface of the metal,

and continuing the electrolysis until an obsorbent, adherent, fiatcoating is produced on such surface.

3. The method of forming corrosion resistant coatings on aluminum whichincludes the steps assaesa of subjecting the aluminum to electrolysis asan anode in an aqueous solution of a potential condensation product offormaldehyde and a material of the class consisting of urea and phenol.and having a film forming electrolyte added thereto, and carrying outthe electrolysis at a voltage greater than the breakdown voltage of thefilm.

4. The method of forming corrosion resistant coatings on film formingmetal from the group consisting of aluminum, magnesium, tantalum andtheir alloys, which includes the steps of subjecting the metal toelectrolysis as an anode in an aqueous solution of a potentialcondensation product of formaldehyde and a material of the classconsisting of urea and phenol, and having a film forming electrolyteadded thereto, and carrying out the electrolysis at a voltage such thatsparking takes place on the surfaces of the metal.

5. The method of forming corrosion resistant coatings on aluminum pieceswhich comprises simultaneously subjecting a plurality of pieces toelectrolysis as an anode in an aqueous solution of a film formingelectrolyte and a potential condensation product of urea andformaldehyde at such a voltage that sparking takes place on the surfacesof the pieces, and keeping the pieces in motion and in loose contactwith a source of current during the electrolysis.

6. The method of forming a corrosion resistant coating on piecescomposed of film forming metal from the group consisting of aluminum,magnesium, tantalum and their'alloys which comprises simultaneouslysubjecting a plurality oi pieces'to electrolysis as an anode in anaqueous solution of a film forming electrolyte and an organic potentialresinous condensation product of the formaldehyde type at such a voltagethat sparking takes place on the surfaces of the pieces, and keeping thepieces in motion and in loose contact with a source of current duringthe electrolysis.

7. The method of forming a corrosion resistant coating on aluminumpieces which comprises simultaneously subjecting a plurality of piecesto electrolysis as an anode in an aqueous solution of a film formingelectrolyte and an organic potential resinous condensation product ofthe formaldehyde type at such a voltage that sparking takes place on thesurfaces of the pieces, and keeping the pieces in motion and in loosecontact with a source of current during the electrolysis.

8. Aluminum having an electro-formed coating of an oxide film containingaluminum and oxygen thereon, said coating being adherent, corrosionresistant and absorbent, presenting a fiat, substantially white,non-metallic appearance, and being produced by subjecting the aluminumto electrolysis as an anode in an aqueous solution of a film formingelectrolyte and a potential condensation product of formaldehyde and amaterial of the class consisting of urea and phenol, and carrying outthe electrolysis at a voltage greater than the breakdown voltage of thefilm.

JOSEPH B. BRENNAN. LEONA MARSH.

